Latex binders are used in a wide variety of nonwoven and textile applications. In nonwovens, they are used to bind together a loosely assembled mass of fibers to form a self-sustaining web which can then be used to produce disposable diapers, consumer towels, disposable wipes, filtration products and the like. In textiles, they are used, for example, in fiberfill, upholstery backcoating, ticking coatings, flocking adhesives and high pile coatings. Depending upon the specific end use, the binders are formulated with other ingredients such as fillers, pigments, foaming agents, foam stabilizers, catalysts, thickeners and the like.
For many of these applications, it is desired to incorporate crosslinking monomers into the latex which, after application to the substrate, will crosslink in a weakly acidic pH range or in the presence of latent acid catalysts at elevated temperatures in order to improve the durability of the textile or nonwoven. Monomers derived from the reaction of a polymerizable amide with an aldehyde are conventionally utilized for such purposes. Generally, these N-alkylol amide containing monomers correspond to the structure: ##STR1## where Y is ##STR2## and where R.sub.1 is H or CH.sub.3 ; R.sub.2 is H or CH.sub.3 ; n is 0 to 3; R is H or CH.sub.3 or ##STR3## where R.sub.3 is C.sub.1 to C.sub.4 alkyl or ##STR4## and x is 2 to 4.
Of these monomers, N-methylol functional ethylenically unsaturated monomers are most often used. The N-(2,2-dialkoxy-1-hydroxy)ethyl acrylamide monomers are also examples of frequently used monomers of this type.
Although N-methylol acrylamide (NMA) is the most widely used crosslinking monomer in such applications, it is also a recognized source of formaldehyde, a chemical the presence of which manufacturers are seeking to eliminate or reduce. Thus, NMA is generally prepared by the reaction of a 1:1 molar mixture of aqueous acrylamide and formaldehyde under alkaline conditions (pH 9-10), a reaction leading to an equilibrium mixture containing NMA, acrylamide and formaldehyde in a ratio of approximately 96, 2.5 and 1.4%, respectively.
Previous attempts to shift the equilibrium towards lower free formaldehyde by increasing the pH of the reaction have led to undesirable side reactions such as polymerization of the acrylamide monomer or destruction of formaldehyde via the Cannizarro reaction. Another approach to lowering the free formaldehyde level is disclosed in U.S. Pat. No. 4,449,978 and involves using a molar excess of acrylamide in emulsion polymers prepared with NMA. Various scavengers have also been suggested, such as are taught in U.S. Pat. No. 5,143,954. However, the presence of these scavengers or their reaction products is not always desirable in the final product.
The amount of free formaldehyde present in emulsion binders which contain N-methylol acrylamide is quantifiable in three different aspects. The first is the free formaldehyde in the emulsion polymer as used. The second measurement is the free formaldehyde emitted by the emulsion polymer upon impregnation, coating, drying and curing in the nonwoven or textile product. The third measurement is free formaldehyde present in the finished nonwoven or textile material. The formaldehyde level in the final product is especially important in binders and coatings which are to be in direct contact with human skin such as in the case of baby diapers, paper towels and the like. The amount of formaldehyde emitted upon drying is also important in the conversion of large fabric rolls where the levels of formaldehyde which may be released into the work environment are limited.
When N-(2,2-dialkoxy-1-hydroxy)alkyl acrylamide monomers such as N-(2,2-dimethoxy-1-hydroxy)ethyl acrylamide (DMHEA) are used as the crosslinking monomer in latex binders, it is possible to produce binders which are entirely free of formaldehyde. However, since the reaction to produce the monomer also results in an equilibrium mixture, the presence of the free aldehyde component has been found to produce undesirable discoloration in the dried webs bonded therewith. It is therefore desirable to minimize the level of free dialkoxyacetaldehyde in the DMHEA monomer.